The subject invention is directed generally to multiple approximation analog-to-digital converters (ADCs), and is directed more particularly to an automatically gain controlled multiple approximation ADC with automatic gain control (AGC) and gain controlled amplifier (GCA) functions implemented in the feedback or feed forward loop or loops of the multiple approximation ADC. Multiple approximation ADCs are also referred to as sub-ranging ADCs, multiple pass ADCs, and successive approximation ADCs.
ADCs are utilized in digital signal processing systems wherein an analog signal is converted to a digital signal for further processing. For example, ADCs are utilized in radio frequency receivers such as broadcast radio receivers, radar receivers, and the like.
In radio frequency receiver applications of ADCs, gain controlled amplifiers (GCA's) are used prior to analog to digital conversion in order to minimize the signal to noise ratio (SNR) degradation caused by the analog to digital conversion process. The output SNR degradation in an ADC drops proportionally to the input signal power, so it is optimal to have the input signal power be as large as possible. On the other hand, the ADC is also a clipper, meaning that it has a maximum input signal range prior to clipping, after which SNR degrades drastically. Therefore, AGC amplification is used to optimize SNR.
Considerations with gain controlled amplifiers include AGC loop instability due to loop delays and gain; inadequate AGC attack speed, in reference to how fast the AGC can track a dynamic input signal, which can result in suboptimal SNR results; and suboptimal GCA linearity for some gain settings, since optimization can generally be performed over some given gain range.
A linear predictive coding ADC is a form of a multiple approximation ADC which has been proposed in an effort to overcome dynamic range limitations. See, for example, "Developments in Techniques for Enhancing The Dynamic Range of Analog to Digital Converters," McKnight et al., Proc. of ICASSP 1988, pages 1742-1745, which is directed to an implementation technique of a bandlimited predictor that provides improved performance in the presence of quantizing noise.
An important consideration with the LPC ADC discussed by McKnight, et al. is the potential for loop instability. The loop can be driven into undesirable limit-cycle oscillations (LCO's) under some conditions. An LCO is seen as a toggling between maximum positive full scale output and maximum negative full scale output which is sustained until the LPC ADC is subjected to a reset sequence, wherein the input is significantly reduced in power and the predictor is zeroed wherein all memory is erased. The cause for the LCO is two-fold: 1) the inability of the predictor to predict the next sample with enough accuracy due to excessive input signal slew rate, such that the error voltage at the summing amplifier causes the internal quantizer to clip; and 2) the input signal amplitude exceeds the maximum full-scale range of the digital-to-analog converter (DAC) which converts the digital prediction value to analog. This latter also causes clipping from the DAC, which results in unacceptable error at the quantizer input causing further clipping and hence LCO.